1. Field of the Invention
The present invention relates to a zoom lens system of a high zoom ratio of about 3, ranging from a telephoto to a considerably wide field angle for use in a still camera, such as 35 mm SLR cameras.
2. Description of the Prior Art
In this field of art, there has been various attempts to provide a zoom lens system of a relatively high zoom ratio ranging to wide field angle. Prior art zoom lens systems provided through such attempts can be roughly classified into two types, the first one having a negative front lens group, while the second one having a positive front lens group.
The first type is in its nature favorable to the purpose of providing a wide angle zoom lens system ranging to a considerably wide field angle, while it is unfavorable to the reduced total length and the compactness of the lens system. A most popular and simplified zoom lens system classified into the first type is a so-called two-group zoom lens system consisting of a negative front lens group and a positive rear lens group. Although such a two-group zoom lens system is suitable for a limited zoom ratio, such as 2, it is difficult to further increase the zoom ratio.
On the other hand, in the second type of zoom lens system with the front positive lens group, if the zoom range is extended to reach a focal length less than the length of a diagonal line across the effective image plane, focusing by means of moving the front positive lens group is difficult or the diameter of the front positive lens group excessively increased. A typical example of a zoom lens system classified into the second type is a so-called four-group zoom lens system consisting of a front focusing lens group, a variator lens group, a compensator lens group and a relay lens group. In such a four-group zoom lens system, if the zoom range is extended to a focal length of less than 40 mm with the focusing capability provided by means of the front lens group and the compactness of the whole lens system maintained, the focal lengths of the front lens group and the variator lens group become excessively short and cause various aberrations which are difficult correct.
An example of a zoom lens system of a high zoom ratio ranging from a telephoto to a wide field angle is disclosed in Japanese Patent Publication No. 55-14403.
The prior art is still seeking to provide a relatively compact and economical zoom lens system having a relatively high zoom ratio ranging to the wide angle range with an adequate optical correction.